Add version to installer-path variables

[bwabt]

Feature request: Some libraries are using versions as sub-paths (e.g. libraries/{$name}/{$version}), presumably to allow for multiple version support. If its easy to expose this as a variable, then this would save some code.

[Seldaek]

Multiple version support makes no sense in a PHP context IMO as two same classes of different versions cannot coexist, so I don't quite get what the point is here.